High-power and high-beam-quality terahertz (THz) quantum cascade laser (QCL) as an emerging THz solid-state radiation source are attracting attentions for numerous applications including medicine, sensing, and communication. However, due to the sub-wavelength confinement of the waveguide structure, direct beam brightness upscaling with device area remains elusive due to several mode competition and external optical lens is normally used to enhance the THz beam brightness.

In a new paper published in Light Science & Application, a team of scientists, led by Professor Quanyong Lu from Beijing Academy of Quantum Information Sciences, and Professor Fengqi Liu from Institute of Semiconductors, CAS surmount these challenges by developing large-scale terahertz quantum cascade surface-emitting lasers with controlled losses and couplings inside the metallic THz phase-engineered photonic crystal (PEPC) to suppress the oscillation of higher-order modes, and by adjusting the phase and shape relationships between the elements of the photonic crystal (PC) lattice, precise control of optical losses among the cavity modes and band-edge modes is achieved. Even with a significant increase to device size (~1.6 mm × 1.6 mm), single mode, single THz spot output is still able to be achieved, thus enabling high-brightness output from this type of laser. The quantum cascade surface-emitting laser is capable of delivering an output peak power over 185 mW with a narrow beam divergence of 4.4°×4.4° at 3.88 THz. A high beam brightness of 1.6×107 W?sr?1m?2 with near-diffraction-limited M2 factors of 1.4 in both vertical and lateral directions is achieved from a large device area of 1.6×1.6 mm2 without using any optical lenses, as shown in Figure (f). The engineered phase design between the lattices enables a stable and high-intensity surface emission over a broad device area, which makes it an ideal light extractor for broad-area THz emitters.

Their work demonstrates a practical approach to brightness enhancement of electrically pumped THz QCL without using any external optical setup. The high-brightness surface-emitting THz PEPC QCLs would open up many new applications in standoff THz imaging, detection, and diagnosis. The results were published in the journal  Light: Science & Applications under the title“High brightness terahertz quantum cascade laser with near-diffraction-limited Gaussian beam”on Aug. 16th, 2024.

Figure | Scheme and performance characterization of THz PEPC QCLs.

(a) Schematic diagram of a THz PEPC QCL structure with non-etched active region scheme. The PEPC resonators was constructed by using metal and air interface, adopting a non-etched active region scheme. (b) Calculated in-plane 1D and 2D coupling coefficients as a function of phase shift d between the two lattices. (c) Modal loss margin between the fundamental and first high-order modes in the PEPC resonator as a function of phase shift. The inset shows the high-order mode profile involving with the phase shift. The dashed line indicates the modal loss margin of a PC cavity with similar lattice parameters. The shaded area indicating the phase shift region with the insufficient in-plane 1D coupling strength for the given device area. (d) L-I-V characterization of a THz PEPC QCL with d=0.3a and L=1.6 mm at different temperatures under a pulsed condition with a repetition rate of 10 kHz and a wide pulse of 4-μs duration. The I-V curve was taken at 13 K. (e) Spectrum of THz PEPC QCL (d=0.3a) near the roll-over current at 13 K, and the measured SMSR is over 20 dB. (f) Experimental 2D far-field emission pattern of the THz PEPC QCL, where θx and θy are angles with respect to the surface normal along the longitudinal and lateral directions of the PEPC, respectively.

This research received funding from National Natural Science Foundation of China , and Beijing Municipal Science & Technology Commission.

The Light: Science & Applications will primarily publish new research results in cutting-edge and emerging topics in optics and photonics, as well as covering traditional topics in optical engineering. The journal will publish original articles and reviews that are of high quality, high interest and far-reaching consequence.

Paper Link: https://www.nature.com/articles/s41377-024-01567-2